Acoustic wave devices are built on piezoelectric substrates, which convert electrical energy into mechanical energy and vice versa. Acoustic wave devices are used in a variety of ways. For example, radio frequency (RF) circuits often use acoustic wave devices as RF filters. The RF filters may be formed by one or more interdigital transducers (IDTs) built on a piezoelectric substrate. An RF input signal may be received by an IDT, which induces a mechanical response in the piezoelectric substrate that causes the piezoelectric substrate to stretch and compress thereby propagating acoustic waves. These propagating acoustic waves cause the piezoelectric substrate to generate a voltage based on the longitudinal and/or shear vector components of the acoustic waves. In turn, these voltages generate a filtered RF output signal on the IDT or on another IDT built on the piezoelectric substrate. Acoustic wave devices may also be utilized to form other types of electronic devices in RF circuits such as resonators, sensors, transformers, and the like.
To maintain these acoustic wave devices from being damaged, acoustic wave devices need to be protected from moisture, temperature variations, and other environmental conditions. Acoustic wave devices are thus often provided in hermetically sealed packages. Unfortunately, hermetically sealed packages may be expensive, difficult to manufacture, and have limited life spans. Also, these hermetically sealed packages can significantly increase the volume consumed by the acoustic wave device.
Rather than using hermetically sealed packages, it would be desirable to provide a protective film over the acoustic wave device. Protective films have been shown to effectively protect other types of electronic devices from environmental conditions. However, sputtering deposition processes and chemical vapor deposition (CVD) processes form protective films that are too thick for acoustic wave devices. The thickness of these protective films shift the resonant frequency and introduce unacceptably high insertion losses into the transfer function of the acoustic wave device. Furthermore, these protective films may have temperature expansion coefficients that are significantly different than those of the metallic components of the acoustic wave device. In turn, this may damage the protective coating and thus allow moisture to penetrate the protective coating.
Finally, the aforementioned deposition techniques do not provide protective films with sufficient uniformity. For instance, sections on the surface of the acoustic wave device may have high aspect ratios and create shadow areas that do not receive as much protective material during the sputtering deposition process or the CVD process. These shadowed areas may cause significant variations in the thickness of the protective film, which further alters the transfer function of the acoustic wave device, and may cause the protective film to have pin-holes and voids that expose the acoustic wave device to moisture and other environmental conditions.
Thus, a protective film is needed on the acoustic wave device that is thinner and more uniform than those provided by the aforementioned deposition processes. Also needed are methods of forming this thinner and more uniform protective film on the surface of the acoustic wave device.